Group notifications is a very exciting feature. This allows you to setup groups for users to opt into so you can be more specific with your push notifications.
You'll start by logging into your App Dashboard, the selecting Push Messages, here you'll see a dropdown list and one of them is Groups. Once you select groups you'll be brought to the following page(screenshot below).
From here you can select Add New Group and a popup will appear (screenshot below) for you to input your group name and description(if needed). Keep in mind the description is only going to be shown on your dashboard and not to the end user. Once complete select submit.
You can add as many groups as you like. You can also edit the name of the group. Keep in mind, because of users that may be subscribed to that group, you need to keep that group purpose the same if you go in and change it in any way.
Once you have your groups setup you're ready to send notifications! Go back to the drop down on the left that says Push Messages, select Send New, fill in the notification like normal and then select Filter By Groups at the bottom(screenshot below).
A popup will appear with all of your groups(screenshot below). Select the group(s) that apply and then click select. Then you're ready to push send! Then only those who have notifications selected for the group(s) will receive it!